This is something I had heard before, but didn't really think about it until a reader I was helping ran into it. It involves Safari on iOS (and I believe just iOS 5 and higher). If you have a web page using timed JavaScript (for example, a setInterval), and switch to another tab (or another app), then the operating system will pause the JavaScript environment. Here is an incredible simple way to see this in action.

The code simply updates a div with a timestamp and an incrementing value. If you open this in your web browser on an iDevice, note the current value, and switch over to another tab, you'll see when you come back that the page had been paused in the background.

I tried to get around this by adding a focus/blur event handler to my code...